The first two characters in each column are the characters you have to type to
enter the digraph.
In the middle of each column is the resulting character.
The decimal number is the normal Unicode number of the character.

It is also possible to enter any character (which can be displayed in your current 'encoding'), even a character for which no digraph is defined, if you know the character value, as follows (where ^V means "hit Ctrl-V, except if you use Ctrl-V to paste, in which case you should hit Ctrl-Q instead):

Vim knows how to work with combining characters as well as more traditional self-contained characters. A combining character is a character you enter separately from a preceding base character, which changes its meaning and is drawn in the same space. For example, 'ä' could be entered either as a single 'ä' character, or as an 'a' character followed by a combining ' ¨ ' character. By default, Vim accepts up to two combining characters for a given base character. You can use the 'maxcombine' option if you need to use more.

You can enter combining characters using either of the methods given above, if you know the byte value or have a defined digraph. Once entered, however, it can be confusing to work with, since Vim mostly treats the base character and combining character as a single character. Using the 'delcombine' option, you can work with combining characters separately in some contexts.